“Linux CUPS Printing Package”
Official xerox support. Probably comes as a compressed Tar file, just move it to your CUPS folder and expand it. The Linux will find it when needed. CUPS is where all printer drivers go so that Linux can find them.
https://www.support.xerox.com/en-us/product/phaser-6200/downloads
Even Xerox support & drivers returns “No results for phaser6125”.
Believe me, I have searched for years. Many support forums of all kinds have phaser6125 owners trying to find drivers. The phaser6125 was apparently an el-cheapo special production with no support beyond Win7.